USING RADAR STRATIGRAPHIC ANALYSIS TO IDENTIFY EROSION AND DEPOSITION IN THE DULUTH BAY BARRIER, LAKE SUPERIOR

The Duluth Bay Barrier protects the major Great Lakes shipping ports of Duluth, MN and Superior, WI from Lake Superior. The barrier is divided into Park Point and Wisconsin Point by a natural inlet. Over 2.5 km of shore parallel and shore perpendicular ground penetrating radar (GPR) transects were collected and analyzed on both points. GPR uses electromagnetic (EM) to infer a subsurface image. Common midpoint surveys were collected and a near surface velocity of .079 m/ns was deduced, allowing more accurate depth calculations. This study utilized a pulseEKKO 100 GPR system for data collection with 100 MHz antennae. Data was processed and plotted through pulseEKKO software. Radar stratigraphic analysis divides reflection patterns into radar facies based on changes in geometric characteristics. Interpretation of radar facies allows the evolution of the Duluth Bay Barrier to be reconstructed. A major, continuous, undulating reflection is imaged between 4-6m depth and interpreted as an erosional surface created during a lower lake levels. In shore parallel transects, facies are dominated by northward dipping reflections suggesting that littoral drift from the southeast feeds the Duluth Bay Barrier. Predominate radar facies in shore perpendicular transects are lakeward dipping inclined to sigmoidal reflections interspaced with subhorizontal reflections. The pattern is interpreted to be repeated erosional and depositional phases. Sigmoidal to inclined reflections are interpreted as erosional beach faces, subhorizontal reflections are believed to result from deposition in the surf zone. Radar stratigraphic analysis shows how erosion, littoral drift, and human activity have affected the Duluth Bay Barrier.